1. Technical Field
The invention relates generally to ohmic contacts, and more specifically, to an improved ohmic contact for a nitride-based semiconductor device.
2. Background Art
A challenge in developing high power III-V nitride-based semiconductor devices, such as light emitting diodes, laser diodes, bipolar junction transistors, and heterojunction bipolar transistors, is the development of an ohmic contact that has both a low specific resistance and a high current carrying capability. Magnesium (Mg), with a room-temperature activation energy as high as two hundred fifty meV, is a commonly used acceptor for p-type Gallium Nitride (GaN) semiconductors. To this extent, some approaches seek to develop an ohmic contact for a Mg-doped p-type GaN semiconductor. However, a relatively low p-doping (e.g., less than 1×1018 cm−3) in p-type GaN, which is achievable either by metalorganic chemical vapor deposition or molecular beam epitaxy, makes the formation of such p-type ohmic contacts difficult.
The activation energy for Mg increases almost linearly as Aluminum (Al) is added to form Mg-doped AlGaN ternary semiconductors, which are used in semiconductor devices such as deep ultraviolet light emitting diodes (UV LEDs). Consequently, several approaches have been suggested to enhance the Mg-doped AlGaN p-type conductivity. In one approach, a Mg-doped AlGaN/GaN short period superlattice (SPSL) replaces the p-type AlGaN semiconductor in the semiconductor device, such as a 340-350 nm UV LED. In this approach, the period of the SPSL is typically below four nanometers. Since minibands are formed in the SPSL, vertical conduction of the p-type SPSL should not be degraded.
In another approach, a Mg-doped AlGaN/GaN large period superlattice (LPSL) is used. In this approach, the period is typically larger than fifteen nanometers and the valence band discontinuity as well as the polarization fields can enhance the ionization of the acceptors in the AlGaN barriers and transfer holes into GaN wells. However, the large period inhibits wavefunction coupling between neighboring wells, which reduces the vertical conductivity. As a result, the LPSL approach can only achieve good horizontal p-type conductivity.
In still another approach, a p-type GaN/p-type AlGaN single heterostructure is used to achieve hole accumulation at the interface. The mechanism of this approach is similar to the LPSL approach. However, since only a single barrier exists for hole transportation, the vertical conductivity is greatly enhanced due to a high-density hole accumulation at the interface, field assisted tunneling, as well as thermal emission. Many deep UV LEDs use this approach for hole injection layers and obtain reasonably good power.
Using the last approach, p-type contact resistivity of 1.1×10−6 ohm-cm2 has been achieved. In particular, a Palladium/Silver/Gold/Titanium/Gold (Pd/Ag/Au/Ti/Au) metallic contact was used under high-current operation for a vertically conducting GaN/InGaN multiple quantum well LED structure grown on a Silicon Carbide (SiC) substrate. However, ohmic contacts to p-type nitrides remain a problem. Particularly for AlGaN compounds with a high Al molar fraction.
As a result, a need exists for an improved ohmic contact. In particular, a need exists an improved ohmic contact for a nitride-based semiconductor device that addresses one or more of these limitations and/or other limitation(s) not expressly discussed herein.